Human immunodeficiency virus (hereinafter referred to HIV) which causes AIDS produces a precursor protein comprising Gag protein used for the formation of the said virus particles and reverse transcriptase in host cells. This precursor protein is cleaved by a protease (hereinafter referred to HIV protease) derived from the virus into a specific size to perform its function. Therefore, an HIV protease inhibitor exhibits antiviral activity by inhibiting an enzymatic activity of HIV protease to block the formation and maturation of infectious virus particles. Several kinds of HIV protease inhibitors have been already reported, comprising synthetic peptide-like compound called as a transition-state mimetic (T. Robins, J. Plattner, J. Acquir. Immun. Defic. Syndr. 6, 162 (1993)). Hydroxyethylamine type derivatives such as Ro 31-8959 comprising phenylalanine .psi.[CH(OH)CH.sub.2 N] decahydroisoquinoline carboxylic acid skeleton similar to the amino acid sequence -Tyr . . . Pro- or -Phe . . . Pro- as a cleavage site of the HIV protease (N. A. Roberts et al., Science 248, 358-361 (1990)) and hydroxymethylcarboxamide type derivatives such as peptide derivatives comprising a norstatine skeleton phenylalanine .psi.[CH(OH)C(O)N] proline were reported to be useful as a HIV protease inhibitor (T. F. Tam et al., J. Med. Chem. 35, 1318-1320 (1992)).
The present inventors also found that a group of synthetic peptides which were transition-state mimetic comprising 3-amino-2-hydroxy-4-phenylbutanoyl residue as the skeletal structure thereof strongly inhibited HIV protease activity to be useful as an anti-AIDS agent and proposed them as HIV protease inhibitors (Japanese laid-open patent No. 170722/1993).
These transition-state mimetics are considered as the most promising anti-AIDS agent of next generation following reverse transcriptase inhibitors of nucleic acid derivatives, such as AZT (azide thymidine), DDC (dideoxycytidine), DDI (dideoxyinosine), which are already used clinically as anti-AIDS agents and clinical use, clinical tests and researches thereof are in progress. That is, clinical application of HIV protease inhibitors has been tried to suppress the formation of virus particles in host cell and prevent the proliferation and infection of HIV, resulting in the prevention of onset of AIDS (Nakajima et al., Gekkan-Yakuji, vol. 35, 2983-2989 (1993)).
However, among these peptide-like compounds, conventional-type of compounds belonging to hydroxymethylcarboxamide derivatives exhibiting excellent HIV protease inhibitory activity have hydrophobic acyl group at N-terminal amino group of tripeptide chain. Therefore, in many cases, problems, such as, (1) their insolubility in water, (2) unstability in vivo, (3) low oral absorptivity have been reported (Hiroaki Mitsuya, Kagaku, vol. 64, No. 7, p462-470 (1994)). Since anti-AIDS agents are consecutively administered for long duration, development of compound with higher bioavailability, that is, easily absorbed and stable in vivo, especially in the case of oral administration has been desired. Development of a peptide compound with excellent HIV protease inhibitory activity which has a low molecular weight and is resistant against degradation by various kinds of digestive enzymes or proteolytic enzymes, is desired. More specifically development of a novel peptide compound with a small size of acyl group linked to N-terminal amino group which comprises only low molecular weight dipeptide-structure as transition-state mimetic is desired.